It is common to filter a flow of melted plastic in the high pressure line leading to an extrusion die or injection molding cavity to insure high quality of the resulting plastic products.
Melted plastic represents a unique problem insofar that standard fluid filtering techniques are concerned by virtue of its substantial viscosity, the high temperatures at which plastic must be maintained, the narrow range of temperatures in the elevated temperature range in which plastic must be maintained to insure satisfactory flow without high temperature degradation, the extremely high pressures under which plastic is subjected to cause the flow, and the need for a homogeneous flow that not only is devoid of contaminants but of air and other gases as well.
Some of these problems are insubstantial where prime virgin plastic is used in the production run, but they are compounded for plastic of lesser quality such as recycled plastic.
Recycled plastic represents a valuable resource if it can be filtered or cleaned sufficiently before extrusion. Its quality can be greatly enhanced if it is carefully filtered to remove all of the contaminants associated with recycled materials, which include degradations of the plastic itself and all types and sizes of nonplastic particulate matter. Recycled plastic, however, creates a particularly significant problem because of the size and volume of contaminants to be removed, and the need for frequent purging of the filtering means.
Many plastic filtration devices filter efficiently, but purging requires disassembly of the unit and significant downtime while the filter screen is cleaned or replaced. In the case of recycled plastic, where the contaminants are considerable, filtering apparatus without some type of intermittently operating purge mechanism cannot effectively be used.
Other types of plastic filters include purge mechanisms, but they are usually either of insufficient capability to handle the contaminants filtered from recycled plastic, or the purge itself involves a significant waste of material during the purging process. For example, it is known to use a portion of the filtered material as a reverse flow through the filter to dislodge contaminants, but the flow of filtered plastic required for the purge is either substantial, or, if a lesser volume, insufficient to accomplish the purge.
In all cases involving the purge of filtering apparatus from melted plastic, it is essential to either avoid the entry of air or other gases into the flow or to run the plastic a period of time after the purge is completed to avoid the presence of air bubbles in the product, which makes it unacceptable.